CMake append objects from different CMakeLists.txt into one library

Question

I would like to create a single library from objects from multiple sub-directories, each one containing their own CMakeLists.txt with OBJECT library trick to have multiple t

Answer 1

Several OBJECT libraries can be incorporated into one INTERFACE library:

# Create lib1 OBJECT library with some sources, compile flags and so.
add_library(lib1 OBJECT ...)

# Create lib2 OBJECT library with some sources, compile flags and so.
add_library(lib2 OBJECT ...)

# Create INTERFACE library..
add_library(libs INTERFACE)
# .. which combines OBJECT libraries
target_sources(libs INTERFACE $ $)

Resulted library can be used with standard target_link_libraries:

add_library(mainLib )
target_link_libraries(mainLib libs)

It is possible to combine INTERFACE libraries futher:

# Create libs_another INTERFACE library like 'libs' one
add_library(libs_another INTERFACE)
..

# Combine 'libs' and 'libs_another' together
add_library(upper_level_libs INTERFACE)
target_link_libraries(upper_level_libs INTERFACE libs libs_another)

Resulted libraries' "chain" could be of any length.

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While this way for combination of OBJECT libraries looks hacky, it is actually allowed by CMake documentation:

Although object libraries may not be named directly in calls to the target_link_libraries() command, they can be "linked" indirectly by using an Interface Library whose INTERFACE_SOURCES target property is set to name $.